This invention relates to brushless synchronous generators in general and more particularly to an improved protective device for a brushless rotating synchronous generator with an exciter.
Brushless synchronous generators are known in which the synchronous generator contains a three phase armature winding in its stator and has its field winding in the rotor. Co-rotating therewith is a three phase exciter armature with the voltage from the exciter armature supplied through rectifiers to energize the rotating field winding of the synchronous generator. The exciter is energized by a voltage taken from the three phase winding of the synchronous generator which, after rectification, is controlled by a series regulator. The series regulator is used to control the exciter field such that the synchronous generator is excited to provide a constant output voltage. A bypass diode is coupled in parallel across the exciter field winding.
Typically in such devices series regulation using a transistor regulator as a constant voltage generator, for example, is employed. With such voltage regulation and, if there is a defect in the regulator circuit, the full voltage from the synchronous generator can be applied to the exciter field winding which in turn causes an overvoltage to be applied to the field winding of the synchronous generator. As a result the machine voltage rises to a overvoltage value which is limited only by the saturation of the magnetic circuit. Such an overvoltage can be up to more than a 150% of the nominal voltage. In order to protect the consumers of the electricity supplied by the synchronous generator from such harmful overvoltages resulting from defects, the synchronous generator must be de-energized where such a defect occurs. However, on the other hand, de-energization must be prevented where an overvoltage occurs simply due to operational reasons such as, for example, switching off at full load.